A semiconductor monocrystal is a substance with which specific atoms are aligned in a periodic and regular manner. Its electron propagation characteristics are determined by the atomic interval inside the semiconductor crystal. That is, a semiconductor has an energy band gap, and this energy band gap is determined by the wave properties of electrons and the periodic potential of the atoms.
Meanwhile, a photonic crystal is a three-dimensional structure wherein substances that exhibit a potential difference with respect to light, in other words, substances with a refractive index difference are aligned in a period close to the wavelength of light. Such substances that make up a photonic crystal have been proposed by Yablonovic and others.
Within a photonic crystal, the optical propagation characteristics are limited by the constraints of the wave properties of light. That is, the propagation of light inside a photonic crystal are subject to restrictions in a manner similar to the propagation of electrons in a semiconductor. In a photonic crystal, a forbidden zone for light or so-called photonic band gap exists, and due to the existence of this band gap, light of a specific wavelength band cannot propagate inside the crystal.
Various photonic crystals have been proposed since priorly. For example, there are photonic crystals wherein submicron particles are aligned in a period close to the wavelength of light. For microwave bands, there are photonic crystals in which polymer spheres are aligned within space as the particles.
Besides these, there are photonic crystals with which polymer spheres are hardened inside a metal and thereafter the polymer spheres are dissolved chemically to form periodic microscopic spaces inside the metal, photonic crystals with which holes are bored at equal intervals within a metal, photonic crystals, with which regions that differ from their surroundings in refractive index are formed in a solid material using a laser, photonic crystals, with which a photopolymerizing polymer is processed to a groove-like form using a lithography technique, etc. A photonic crystal that has been formed by such processing has a photonic band gap that is uniquely determined by the structure.
An optical device using such a photonic crystal can selectively output a predetermined wavelength range of input light. In the description that follows, light that is input into a photonic crystal shall be referred to as “input light” and light that is output from a photonic crystal upon passage through the photonic crystal shall be referred to as “output light.”